Conductor gallop is the high-amplitude, low-frequency oscillation of overhead power lines due to wind.
[1] The movement of the wires occurs most commonly in the vertical plane, although horizontal or rotational motion is also possible.
The natural frequency mode tends to be around 1 Hz, leading the often graceful periodic motion to also be known as conductor dancing.
[2][3] The oscillations can exhibit amplitudes in excess of a metre, and the displacement is sometimes sufficient for the phase conductors to infringe operating clearances (coming too close to other objects), and causing flashover.
[4] The forceful motion also adds significantly to the loading stress on insulators and electricity pylons, raising the risk of mechanical failure of either.
The mechanisms that initiate gallop are not always clear, though it is thought to be often caused by asymmetric conductor aerodynamics due to ice build up on one side of a wire.
The crescent of encrusted ice approximates an aerofoil, altering the normally round profile of the wire and increasing the tendency to oscillate.
[3] Gallop can be a significant problem for transmission system operators, particularly where lines cross open, windswept country and are at risk to ice loading.
If gallop is likely to be a concern, designers can employ smooth-faced conductors, whose improved icing and aerodynamic characteristics reduce the motion.
[4] Additionally, anti-gallop devices may be mounted to the line to convert the lateral motion to a less damaging twisting one.
Increasing the tension in the line and adopting more rigid insulator attachments have the effect of reducing galloping motion.
[3] The sudden loss of ice from a line can result in a phenomenon called "jump", in which the catenary dramatically rebounds upwards in response to the change in weight.
[1][2] If the risk of trip is high, the operator may elect to pre-emptively switch out the line in a controlled manner rather than face an unexpected fault.
At large wind velocities, the lift and drag induced on the wire are proportional to the square of the wind velocity, but the proportionality constants CL and CD (for a noncircular wire) depend on α:
[10] In principle, the excited oscillation can take three forms: rotation of the wire, horizontal sway, or vertical plunge.
For algebraic simplicity, this article will analyze a conductor only experiencing plunge (and not rotation); a similar treatment can address other dynamics.
[10] Gallop occurs whenever the driving coefficient 1/2ρlU · (CD + ∂CL/∂α)|α = 0 exceeds the natural damping of the wire; in particular, a necessary-but-not-sufficient condition is that
[9][10] At low wind velocities U, the above analysis begins to fail, because the gallop oscillation couples to the vortex shedding.
[10] A similar aeolian phenomenon is flutter, caused by vortices on the leeward side of the wire, and which is distinguished from gallop by its high-frequency (10 Hz), low-amplitude motion.